1. Field of the Invention
The invention relates to a circuit board and a fabrication method thereof. More particularly, the invention relates a circuit board having high circuit density and a fabrication method thereof.
2. Description of Related Art
A conductive channel is an indispensable component of a circuit board when the circuit board technology is applied. The conductive channel can penetrate one or more insulating layers of the circuit board for connecting two adjacent circuit layers or two separated circuit layers of the circuit board. Therefore, the two circuit layers are electrically connected to each other.
FIG. 1A is a partial top view illustrating a conventional two-layered circuit board. FIG. 1B is a cross-sectional view taken along a line segment I-I of the circuit board as depicted in FIG. 1A. Please refer to both FIGS. 1A and 1B. The conventional two-layered circuit board 100 includes an insulating layer 110, two circuit layers 120 and 130, and two conductive channels 140 and 150. The circuit layers 120 and 130 are respectively disposed on an upper surface 112 and a lower surface 114 of the insulating layer 110. The circuit layer 120 has a first conductive trace 122 and a second conductive trace 124, and the circuit layer 130 has a third circuit 132 and a fourth circuit 134. The insulating layer 110 has two through holes 116 and 118 in which the conductive channels 140 and 150 are respectively disposed. The conductive channel 140 is connected between the first conductive trace 122 and the third circuit 132, and the conductive channel 150 is connected between the second conductive trace 124 and the fourth circuit 134.
According to the related art, a plurality of conductive channels and a plurality of through holes accommodating the conductive channels are required to electrically connect conductive traces located in two different circuit layers. Nonetheless, the through holes 116 and 118 bring about the decrease in available layout area of the circuit board 100 and the reduction of layout flexibility of the circuit board 100. Moreover, layout density of the circuit board 100 is unlikely to be improved.